1. Field of the Invention
The present invention relates to a process for the preparation of a flexible thermoplastic composite filament containing continuous fibres.
More specifically, the present invention relates to a process for the preparation of a flexible thermoplastic composite filament consisting of bundles of continuous fibres of any kind, either mineral, organic or metallic, impregnated with a thermoplastic polyolefinic resin in powder form and/or in fibres and covered with an external flexible thermoplastic sheath which is also polyolefinic.
2. Description of the Prior Art
In many applications, formally dominated by the use of metallic materials, composite materials have been proposed basically consisting of a polymeric matrix and an inorganic reinforcement, mainly glass or carbon fibres. In conditions which require particularly high performances, from the point of view of mechanical resistance, composite materials are proposed with a long or, above all, continuous fibre.
The first type of composite material with a long or continuous fibre, developed as an alternative to metallic materials, was characterized in that it had a matrix consisting of thermosetting polymers, of which epoxy resins, unsaturated polyester resins and bismaleimides are typical examples.
End products made of these materials are obtained by impregnating and/or pre-impregnating the fibres with the monomer and/or a prepolymer and subsequently forming the material with technologies such as manual stratification, winding, pultrusion and moulding in a press or autoclave.
In each case the fibres are passed through solutions of the thermosetting resin with consequent problems of environment and process costs due to the necessity of arranging the reuse and/or storage of the solvents used.
To overcome this type of inconvenience, composite materials have quite recently been introduced which use thermoplastic polymers as matrix. In this case the relative transformation technology involves, if the composite is a long or continuous fibre, the preliminary formation of a semifinished product.
An example of the preparation of a semifinished product of this kind is described in U.S. Pat. No. 3,742,106, which discloses the formation of a composite material by the impregnation of a continuous reinforcing filament in a thermoplastic melt.
Another example of semifinished product for composites with a thermoplastic matrix is described in Journal of Applied Polymer Science--Applied Polymer Symposium, vol. 47, page 501, 1991, which discloses the preparation of a continuous filament by the spinning of a mix consisting of reinforcing and thermoplastic fibres.
U.S. Pat. No. 4,614,678 describes the formation of a composite by the impregnation of a bundle of reinforcement fibres with thermoplastic powders and the covering of the bundle with a sheath which is also thermoplastic.
More specifically, the process described in this patent comprises:
unrolling a continuous filament, consisting of a bundle of fibres, for example glass or carbon fibres, from a bobbin; PA1 feeding the filament to a closed chamber, inside which a thermoplastic polymer in powder form is maintained in a fluidized state by means of gas; PA1 opening, inside the chamber, the bundle of filaments basically into single fibres; PA1 drawing the opened filament through the fluidized polymer for a time which is sufficient for particles of polymer to be deposited around the single fibres; PA1 reclosing the filament and enclosing it with a protective flexible sheath of thermoplastic polymer preferably of the same kind as the powder polymer.
After obtaining the semifinished products, units are produced with consolidation techniques which can involve heating and pressure application, as described for example in various items of the Encyclopedia of Composites, prepared by S. M. Lee, published by VCH Publishers, New York, 1990-1991, or the pultrusion of the semifinished products themselves.
The use of thermoplastic composites undoubtedly produces improvements from an environmental point of view as no solvents are involved in the production cycles. Problems do arise however in the transformation into end products, above all when the composite material consists of a continuous glass multifibre filament impregnated and covered with a polyolefin such as polyethylene. In this case, there are problems of compatibilization between the polyolefinic matrix and reinforcement.
It is known, in fact, that to improve adhesion between a reinforcement fibre, particularly glass fibre, and a polymeric matrix, it is necessary to modify the surface of the fibre with substances, for example of the silanic or aminic type, which, by interacting between the reactive sites present on the fibre and reactive groups present in the polymer, produce strong bonds which considerably improve the compatibility between matrix and reinforcement. This surface treatment of the fibre, known as chemical finishing, is amply described in literature, for example in "Reinforced Plastics and Elastomers. Recent Developments", M. W. Ranney, Noyes Data Corporation, 1977, 4.
In the case of the polyolefins, however, owing to the substantial absence of polar or reactive groups in the polymeric chain, the sizing treatment of the fibre alone is not sufficient to guarantee the necessary adhesion between matrix and reinforcement. To overcome this inconvenience a proposal has been made to modify the structure of the olefinic polymer by grafting onto the polymeric chain monomers having polar groups such as maleic acid or anhydride, acrylic or methacrylic acids, etc. The modification operation however has proved to be valid when the polyolefins are mixed in an extruder with short fibres. In this case, in fact, by carrying out the functionalization of the polymer directly in an extruder or by adding a prefunctionalized polymer to the mixture to be treated, it is possible to have a dynamic contact, guaranteed by the extruder itself, between fibre and polymeric matrix which favours the chemical bond between the size of the fibre and the polar groups grafted to the polyolefin.
On the contrary, in the case of composites with a continuous fibre the functionalization of the polyolefin has proved to be ineffective as subsequent transformation technologies of the semifinished products do not permit the above dynamic contact necessary for the compatibilization between reinforcement and polymeric matrix. On the basis of this, the use of polyolefins, and in particular polyethylene, for the preparation of thermoplastic composites reinforced with continuous glass fibres has had little success in the past.